Combustion turbine engines include a compressor assembly, a combustor assembly, and a turbine assembly. The compressor compresses ambient air, which is channeled into the combustor where it is mixed with fuel and burned, creating a heated working gas. The working gas can reach temperatures of about 2500-2900° F. (1371-1593° C.), and is expanded through the turbine assembly. The turbine assembly has a series of circular arrays of rotating blades attached to a central rotating shaft. A circular array of stationary vanes is mounted in the turbine casing just upstream of each array of rotating blades. The stationary vanes are airfoils that redirect the gas flow for optimum aerodynamic effect on the next array of rotating blades. Expansion of the working gas through the rows of rotating blades and stationary vanes causes a transfer of energy from the working gas to the rotating assembly, causing rotation of the shaft, which drives the compressor.
The vane assemblies may include an outer platform element attached to the distal or outer end of the vane. An inner platform element is connected to the inner end of the vane. The outer platform elements are mounted adjacent to each other in a circular array that defines an outer shroud ring attached to a support ring on the turbine casing. The inner platform elements are adjacent to each other to define an inner shroud ring. The outer and inner shroud rings define an annular working gas flow channel between them.
Surrounding each disc of rotating blades is an outer shroud ring assembled as a circular array of arcuate ring segments. The ring segments and vane platforms must withstand high mechanical loads, cyclic stresses, and thermal stresses. They may be made of superalloy metals for strength and ceramic materials for thermal tolerance. For example, a vane platform may be made of a superalloy vane support structure with a ceramic matrix composite (CMC) cover or shroud plate that protects the metal from the combustion gas.